Functional Fun in Kotlin

Kotlin is great language to do modern functional programming, and in my opinion perhaps the best language to do modern mainstream (hardcore) functional programming. With the power of Kotlin DSLs we can make functional programming idiomatic, simple and elegant. This talk takes us through the different techniques that we can apply in Kotlin to achieve modern, and elegant functional patterns.

Transcript

1. Functional Fun in
   Kotlin
   47deg.com
   

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2. Who am I?
   

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3. Functional Fun in Kotlin
   Dependency Injection
   Side effects
   Typed Errors
   

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4. Dependency Injection
   interface Database {
   fun query(sql: String, vararg params: String): ResultRow
   }
   interface Logger {
   fun log(message: String): Unit
   fun error(throwable: Throwable, message: String): Unit
   }
   

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5. Dependency Injection
   fun fetchUser(id: Long, db: Database, logger: Logger): User {
   val user = db.query("SELECT * FROM users WHERE id = $1", id).toUser()
   logger.log("Query result: $user")
   return user
   }
   

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6. Dependency Injection
   fun fetchUser(id: Long, db: Database, logger: Logger): User {
   val user = db.query("SELECT * FROM users WHERE id = $1", id).toUser()
   logger.log("Query result: $user")
   return user
   }
   fun fetchAll(ids: List, db: Database, logger: Logger): List =
   ids.map { id -> fetchUser(id, db, logger) }
   

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7. Dependency Injection
   fun Context.fetchUser(id: Long): User
   

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8. Dependency Injection
   fun Context.fetchUser(id: Long): User
   where Context : Database, Context : Logger
   

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9. Dependency Injection
   fun Context.fetchUser(id: Long): User
   where Context : Database, Context : Logger {
   val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
   log("Query result: $user")
   return user
   }
   

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10. Dependency Injection
    fun Context.fetchUser(id: Long): User
    where Context : Database, Context : Logger {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    class Module(db: Database, logger: Logger): Database by db, Logger by logger
    

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11. Dependency Injection
    fun Context.fetchUser(id: Long): User
    where Context : Database, Context : Logger {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    class Module(db: Database, logger: Logger): Database by db, Logger by logger
    fun Context.fetchAll(ids: List): List
    where Context : Database, Context : Logger =
    ids.map { id -> fetchUser(id) }
    

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12. Dependency Injection
    context(Database, Logger)
    fun fetchUser(id: Long): User
    

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13. Dependency Injection
    context(Database, Logger)
    fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    

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14. Dependency Injection
    context(Database, Logger)
    fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    context(Database, Logger)
    fun fetchAll(ids: List): List =
    ids.map { id -> fetchUser(id) }
    

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15. Side effects
    Compile time tracked effects
    Composable
    Reason about code that
    performs side effects
    

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16. Side effects
    interface Database {
    fun query(sql: String, vararg params: String): ResultRow
    }
    interface Logger {
    fun log(message: String): Unit
    fun error(throwable: Throwable, message: String): Unit
    }
    

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17. Side effects
    interface Database {
    suspend fun query(sql: String, vararg params: String): ResultRow
    }
    interface Logger {
    suspend fun log(message: String): Unit
    suspend fun error(throwable: Throwable, message: String): Unit
    }
    

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18. Side effects
    context(Database, Logger)
    fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    

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19. Side effects
    context(Database, Logger)
    fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    Suspend function 'query' should be called only from a coroutine or another suspend function
    Suspend function 'log' should be called only from a coroutine or another suspend function
    

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20. Side effects
    context(Database, Logger)
    suspend fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    

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21. Suspend - Continuation Passing Style
    interface Continuation {
    val context: CoroutineContext
    fun resume(value: T)
    fun resumeWithException(exception: Throwable)
    }
    

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22. Suspend - Continuation Passing Style
    context(Database, Logger)
    fun fetchUser(id: Long, cont: Continuation): Unit {
    query("SELECT * FROM users WHERE id = $1", id, Continuation(cont.context) { res ->
    })
    }
    

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23. Suspend - Continuation Passing Style
    context(Database, Logger)
    fun fetchUser(id: Long, cont: Continuation): Unit {
    query("SELECT * FROM users WHERE id = $1", id, Continuation(cont.context) { res ->
    res.fold({ resultRow ->
    val user = resultRow.toUser()
    log2("Query result: $user", Continuation(cont.context) { res ->
    })
    }, cont::resumeWithException)
    })
    }
    

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24. Suspend - Continuation Passing Style
    context(Database, Logger)
    fun fetchUser(id: Long, cont: Continuation): Unit {
    query("SELECT * FROM users WHERE id = $1", id, Continuation(cont.context) { res ->
    res.fold({ resultRow ->
    val user = resultRow.toUser()
    log2("Query result: $user", Continuation(cont.context) { res ->
    res.fold({ cont.resume(user) }, cont::resumeWithException)
    })
    }, cont::resumeWithException)
    })
    }
    

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25. Suspend - Continuation Passing Style
    context(Database, Logger)
    fun fetchUser(id: Long, cont: Continuation): Unit {
    query("SELECT * FROM users WHERE id = $1", id, Continuation(cont.context) { res ->
    res.fold({ resultRow ->
    val user = resultRow.toUser()
    log2("Query result: $user", Continuation(cont.context) { res ->
    res.fold({ cont.resume(user) }, cont::resumeWithException)
    })
    }, cont::resumeWithException)
    })
    }
    

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26. Side effects
    context(Database, Logger)
    suspend fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    

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27. Side effects
    context(Database, Logger)
    suspend fun fetchUser(id: Long): User {
    val user = query("SELECT * FROM users WHERE id = $1", id).toUser()
    log("Query result: $user")
    return user
    }
    context(Database, Logger)
    suspend fun fetchAll(ids: List): List =
    ids.map { id -> fetchUser(id) }
    

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28. Compile time tracking of expected
    errors
    Explicit in type signature
    Typed Errors
    

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29. Typed Errors
    context(Database, Logger)
    suspend fun fetchUser(id: Long): User?
    

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30. Typed Errors
    data class UserAlreadyExists(val error: PSQLException)
    context(Database, Logger)
    suspend fun insertUser(name: String, email: String): User
    

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31. Typed Errors
    data class UserAlreadyExists(
    val name: String,
    val email: String,
    val error: PSQLException
    )
    context(Database, Logger)
    suspend fun insertUser(name: String, email: String): Either
    

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32. Typed Errors
    data class UserAlreadyExists(
    val name: String,
    val email: String,
    val error: PSQLException
    )
    context(Database, Logger, Raise)
    suspend fun insertUser(name: String, email: String): User
    

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33. Typed Errors
    context(Database, Logger, Raise)
    suspend fun insertUser(name: String, email: String): User =
    catch({
    query("""
    INSERT INTO users
    VALUES ($1, $2)
    RETURNING user_id;
    """.trimIndent()).toUser()
    }) { error: PSQLException ->
    if (error.isUniqueViolation()) raise(UserAlreadyExists(name, email, error))
    else throw error
    }
    

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34. Monadic DSLs
    context(Database, Logger, Raise)
    suspend fun insertUser(name: String, email: String): User =
    catch({
    query("""
    INSERT INTO users
    VALUES ($1, $2)
    RETURNING user_id;
    """.trimIndent()).toUser()
    }) { error: PSQLException ->
    if (error.isUniqueViolation()) raise(UserAlreadyExists(name, email, error))
    else throw error
    }
    

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35. Arrow
    DSL based functional
    programming
    Idiomatic Kotlin syntax
    Kotlin
    Multiplatform
    ready
    

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36. Saga pattern
    context(SagaScope)
    suspend fun sagaOperation(): Result =
    saga({ action() }) { compensation() }
    

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37. Resource Safety
    context(ResourceScope, Logger)
    suspend fun custom(): CustomResource =
    install({ CustomResource. acquire() }) { res, exitCase ->
    log("Releasing $res due to $exitCase")
    res.release()
    }
    context(ResourceScope)
    suspend fun dataSource(config: HikariConfig): HikariDataSource =
    autoCloseable { HikariDataSource(config) }
    

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38. Thanks!
    

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